How to save an image without antialiasing in cycles?
In 2.67 and 2.68a (I haven't checked 2.69+), the amount of AA present in the cycles render is controlled by the size of the Gaussian sample on the film.

The gaussian for the film is the amount of jitter that rays leaving the camera receive. If this is high, the pixel effectively samples a little circle of incoming light in the scene, and you get anti-aliasing. If this is small, then all the rays are almost the same, and you are sampling one single point on the film.

So to reduce or remove anti-aliasing in Cycles, turn down the Gaussian width in the Film section of the Render settings. Turning it to 0 will reset it to its minimum (currently 0.01 on my version, the default is 1.50), and rendering will give essentially a non AA image.



If you just want to avoid an aliased outline, then this isn't the right setting, do it in the compositor as per PeterT's answer.